You are trying to create a MonoBehaviour using the 'new' keyword.

I am trying to create an inventory system etc and I got stuck on this part when I was making my database.

You are trying to create a MonoBehaviour using the 'new' keyword.  This is not allowed.  MonoBehaviours can only be added using AddComponent().  Alternatively, your script can inherit from ScriptableObject or no base class at all
Item:.ctor(String, String, String, Int32) (at Assets/Scripts/Inventory/Item.cs:14)
ItemDatabase:ItemDatabaseSetup() (at Assets/Scripts/Inventory/ItemDatabase.cs:13)
ItemDatabase:Start() (at Assets/Scripts/Inventory/ItemDatabase.cs:9)

I am trying to make it so I can create items on this list but it isn’t creating all the other things. Any help is really appreciated.

What it looks like now:

scripts involved :

[Links to code samples are a bad way to ask questions. Please include code snippets.]

You have to decide whether your objects are game objects or not. [Visualised or not.]

If they are, then they have to inherit from MonoBehaviour and you have to use AddComponent(), and allow Unity to create the object using its default constructor and serialisation.

If not, then they must not inherit from MonoBehaviour and you can create them using new() any way you like.